Impingement and mixing dynamics of micro-droplets on a solid surface

Abstract

• Impingement on sessile micro-droplet with different impinging angles was simulated. • Convective time, diffusion time and total mixing time were determined quantitatively. • Influence factors on the dimensionless droplet mixing time were discussed. • Mixing time can be shortened by reducing the impinging angle. The hydrodynamics and mixing during the nonaxisymmetry impingement of a micro-droplet and a sessile droplet of the same fluid are investigated by many-body dissipative particle dynamics (MDPD) simulation. In this work, the range of the impingement angle ( θ i ) between the impinging droplet and the sessile droplet is 0°–60° and the contact angle is set as 45° or 124°. The droplets impingement and mixing behavior is analyzed based on the droplet internal flow field, the concentration distribution and the time scale of the decay of the kinetic energy of the impinging droplet. The dimensionless total mixing time ( τ m ) is calculated by a modified mixing function. With the Weber number ( We ) ranging from 5.65 to 22.7 and the Ohnesorge number ( Oh ) ranging from 0.136 to 0.214, we find τ m hardly changes with We and Oh . Whereas, θ i and surface wettability are found to have a significant effect on τ m . We find that θ i has no clear effect on τ m on a hydrophobic surface, while on the hydrophilic surface, τ m increase with the θ i . Thus, reducing the impinging angle is a valid method to shorten the τ m .